The wet oxidation from the mesa sidewalls of AlGaAs/GaAs epitaxial structures is investigated in details. In addition to the intended lateral oxidation of the Al-rich buried layer, we observe a parasitic vertical oxidation of the adjacent layers of lower Al content. This vertical oxidation produces a rough interface between the oxidized and non-oxidized materials and reduces the effective thickness of the adjacent layers. This detrimental phenomenon is drastically reduced when the adjacent layer is replaced by a GaAs/AlAs superlattice (SL). In this case, a sharp interface between the Al-Ga-oxide and the SL is obtained after the oxidation process. Further experiments point out the critical role of hydrogen in the oxidation process of low Al content alloys. Hydrogen atoms produced during the oxidation of the Al-rich layer are proposed to be responsible for the vertical oxidation of the adjacent layers. We devise that the SL interfaces are favorable to reduce the diffusion of the reactants in the vertical direction.